Deprecate `multiunzip` after switching to Rust 1.85

[ronnodas]

Rust 1.85 implements FromIterator for (A_1, ..., A_n) when each A_i is Default + Extend (and n is at most 12) so iter.multiunzip() can be replaced by iter.collect().

[marcospb19]

also deprecate Itertools::unzip, it's for 2-tuples, replaced by .collect() too.

[ronnodas]

There's no Itertools::unzip, maybe you're thinking of Iterator::unzip?

[marcospb19]

Oh you're right, thanks.